This invention relates to a process for forming optical fibers and more particularly to a vaporization process for forming optical fibers.
Optical fibers are currently undergoing intensive development as the transmission link in optical communication systems. In order to perform this function, the fiber should exhibit extremely low optical attenuation and low optical dispersion. The cores of the fibers should be relatively large to facilitate coupling with light sources and splicing between fibers; hence most practical optical fibers are multimode. To reduce modal dispersion in multimode fibers, fibers can be manufactured with refractive indices which decrease gradually from the core to the surface; in this case the higher order modes which travel longer path lengths have higher velocities, thereby allowing all modes to travel with approximately equal axial velocities.
At present, there are two general methods used to prepare graded-index optical fibers; the vapor deposition technique and the double crucible technique. In the vapor deposition method a vapor phase reaction is used to deposit the constituents onto a mandrel. During deposition, the concentrations of the constituents are varied so as to produce the composition gradient required for the desired index profile. After deposition, the preform is thermally sintered and collapsed, and then drawn into a fiber.
One disadvantage of the vapor phase technique is that it is complex and requires sophisticated control during each phase of the process. For example, since the index gradient is introduced in the initial stage of the process, all subsequent steps (i.e., sintering, collapsing and drawing) must be controlled so that no unanticipated changes occur in the gradient.
A further disadvantage of this process is that since the components are co-deposited, they must all be compatible with a single set of deposition conditions; this has largely limited the vapor deposition technique to two-component systems. Consequently, the choice of core and cladding materials is constrained because of secondary property changes which unavoidably occur when primary properties are altered. For example, it is beneficial to increase the refractive index difference between core and cladding to achieve a high numerical aperture; however, the use of two-component glasses restricts the compositional difference between core and cladding because of concommitant changes in expansion coefficient which can give unacceptably high mechanical stresses. Similarly, large compositional differences between the core and cladding may produce viscosity differences which could lead to difficulties during fiber drawing.
The second general method used to prepare optical fibers with graded refractive indices is the double crucible technique. This method consists of first preparing bulk glasses having compositions suitable for use as the core/cladding end-members; these glasses are then remelted and fibers are pulled from concentric double platinum crucibles. The combined glass stream is commonly maintained at a high temperature to permit interdiffusion between the core and cladding in order to provide a graded refractive index profile.
One disadvantage of the double crucible technique is that the index gradient is formed during fiber pulling, which means that simultaneous control is required for both the fiber drawing process and the interdiffusion process. Another disadvantage is that the double crucible technique is prone to impurity problems in the core/cladding interfacial region; specifically a relatively high ingestion of platinum can occur in this area due to partial dissolution and ablation of the drawing crucible.
It would be desirable to provide a method for forming optical fibers which avoids the disadvantages of the prior art, particularly narrow constraints on fiber components, requirements for strict process control and problems associated with impurities or inclusions at the core/cladding interface.